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(#) Implied default locale in case conversion

!!! WARNING: Implied default locale in case conversion
   This is a warning.

Id
:   `DefaultLocale`
Summary
:   Implied default locale in case conversion
Severity
:   Warning
Category
:   Correctness
Platform
:   Any
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   Initial
Affects
:   Kotlin and Java files
Editing
:   This check runs on the fly in the IDE editor
See
:   https://developer.android.com/reference/java/util/Locale.html#default_locale
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/LocaleDetector.kt)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/LocaleDetectorTest.kt)
Copyright Year
:   2012

Calling `String#toLowerCase()` or `#toUpperCase()` **without specifying
an explicit locale** is a common source of bugs. The reason for that is
that those methods will use the current locale on the user's device, and
even though the code appears to work correctly when you are developing
the app, it will fail in some locales. For example, in the Turkish
locale, the uppercase replacement for `i` is **not** `I`.

If you want the methods to just perform ASCII replacement, for example
to convert an enum name, call `String#toUpperCase(Locale.ROOT)` instead.
If you really want to use the current locale, call
`String#toUpperCase(Locale.getDefault())` instead.

!!! Tip
   This lint check has an associated quickfix available in the IDE.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/test/pkg/LocaleTest.java:11:Warning: Implicitly using the default
locale is a common source of bugs: Use toUpperCase(Locale) instead. For
strings meant to be internal use Locale.ROOT, otherwise
Locale.getDefault(). [DefaultLocale]
    System.out.println("WRONG".toUpperCase());
                               -----------
src/test/pkg/LocaleTest.java:16:Warning: Implicitly using the default
locale is a common source of bugs: Use toLowerCase(Locale) instead. For
strings meant to be internal use Locale.ROOT, otherwise
Locale.getDefault(). [DefaultLocale]
    System.out.println("WRONG".toLowerCase());
                               -----------
src/test/pkg/LocaleTest.java:20:Warning: Implicitly using the default
locale is a common source of bugs: Use String.format(Locale, ...)
instead [DefaultLocale]
    String.format("WRONG: %f", 1.0f); // Implies locale
    --------------------------------
src/test/pkg/LocaleTest.java:21:Warning: Implicitly using the default
locale is a common source of bugs: Use String.format(Locale, ...)
instead [DefaultLocale]
    String.format("WRONG: %1$f", 1.0f);
    ----------------------------------
src/test/pkg/LocaleTest.java:22:Warning: Implicitly using the default
locale is a common source of bugs: Use String.format(Locale, ...)
instead [DefaultLocale]
    String.format("WRONG: %e", 1.0f);
    --------------------------------
src/test/pkg/LocaleTest.java:23:Warning: Implicitly using the default
locale is a common source of bugs: Use String.format(Locale, ...)
instead [DefaultLocale]
    String.format("WRONG: %d", 1.0f);
    --------------------------------
src/test/pkg/LocaleTest.java:24:Warning: Implicitly using the default
locale is a common source of bugs: Use String.format(Locale, ...)
instead [DefaultLocale]
    String.format("WRONG: %g", 1.0f);
    --------------------------------
src/test/pkg/LocaleTest.java:25:Warning: Implicitly using the default
locale is a common source of bugs: Use String.format(Locale, ...)
instead [DefaultLocale]
    String.format("WRONG: %g", 1.0f);
    --------------------------------
src/test/pkg/LocaleTest.java:26:Warning: Implicitly using the default
locale is a common source of bugs: Use String.format(Locale, ...)
instead [DefaultLocale]
    String.format("WRONG: %1$tm %1$te,%1$tY",
    ^
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/test/pkg/LocaleTest.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
package test.pkg;

import java.text.*;
import java.util.*;
@SuppressWarnings({"ResultOfMethodCallIgnored", "MalformedFormatString", "MethodMayBeStatic", "ResultOfObjectAllocationIgnored", "SimpleDateFormatWithoutLocale", "StringToUpperCaseOrToLowerCaseWithoutLocale", "ClassNameDiffersFromFileName"})
public class LocaleTest {
    public void testStrings() {
        System.out.println("OK".toUpperCase(Locale.getDefault()));
        System.out.println("OK".toUpperCase(Locale.US));
        System.out.println("OK".toUpperCase(Locale.CHINA));
        System.out.println("WRONG".toUpperCase());

        System.out.println("OK".toLowerCase(Locale.getDefault()));
        System.out.println("OK".toLowerCase(Locale.US));
        System.out.println("OK".toLowerCase(Locale.CHINA));
        System.out.println("WRONG".toLowerCase());

        String.format(Locale.getDefault(), "OK: %f", 1.0f);
        String.format("OK: %x %A %c %b %B %h %n %%", 1, 2, 'c', true, false, 5);
        String.format("WRONG: %f", 1.0f); // Implies locale
        String.format("WRONG: %1$f", 1.0f);
        String.format("WRONG: %e", 1.0f);
        String.format("WRONG: %d", 1.0f);
        String.format("WRONG: %g", 1.0f);
        String.format("WRONG: %g", 1.0f);
        String.format("WRONG: %1$tm %1$te,%1$tY",
                new GregorianCalendar(2012, GregorianCalendar.AUGUST, 27));
    }

    @android.annotation.SuppressLint("NewApi") // DateFormatSymbols requires API 9
    public void testSimpleDateFormat() {
        new SimpleDateFormat(); // WRONG
        new SimpleDateFormat("yyyy-MM-dd"); // WRONG
        new SimpleDateFormat("yyyy-MM-dd", DateFormatSymbols.getInstance()); // WRONG
        new SimpleDateFormat("yyyy-MM-dd", Locale.US); // OK
    }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/LocaleDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `LocaleDetector.testBasic`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=192708.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("DefaultLocale")
  fun method() {
     toLowerCase(...)
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("DefaultLocale")
  void method() {
     toLowerCase(...);
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection DefaultLocale
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="DefaultLocale" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'DefaultLocale'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore DefaultLocale ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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